Apparatus and process for radiation



June 20, 1961 A. T. WILSON 2,989,633

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IN VEN TOR.

Alexander T. Wilson ATTORNEY U i e Staws wfl i Ofl cc 1.

2,989,633 APPARATUS AND PROCESS FOR RADIATION Alexander T. Wilson, Whiting, Ind., assignor to Standd Oil Company, Chicago, 111., a corporation of Indiana Filed Feb. 8, 1956, Ser. No. 564,268 2 Claims. (Cl. 250-495) This invention relates to means for irradiating materials with low energy electrons. It is concerned with the problem of transferring low energy irradiation from the high vacuum environment of electron generation to the surface of materials to be irradiated without excessive energy loss. It is also concerned with the problem of providing means for irradiating thin layers of materials on a large scale at rapid rates using low cost means for generating low energy electrons.

conventionally available electron accelerators depend upon the use of. high voltages, usually in excess of 1,000,- 000 volts. A primary reason for high voltage, in radiation chemistry, is to obtain adequate penetrating power. The beam of electrons must be generated under high vacuum, of the order of about 10- or preferably mm. mercury. After suitable focusing and acceleration, the beam is then passed from the evacuated enclosure through a thin window of aluminum or easily penetrated material, and high voltage is necessary to provide the electron beam with suflicient energy to pass through such windows without losing its capacity for useful irradiation. For example, a 2,000,000 volt electron beam loses only about 50,000 volts of its energy in passing through the window (comprising a few mils of aluminum) of a commercial Van de Graafl. type accelerator. A 250,000 volt electron beam, on the other hand, loses all of its energy in passing through such a window. Thus, electron accelerators operating at voltages such less than about 1,000,- 000 volts are impracticable in a commercially useful sense.

This limits the value of electron accelerators in radiochemical applications because many materials, particularly thermally sensitive materials of organic nature, cannot be exposed to the high energy electron beams produced by conventional accelerators without undesirable break-down or thermal transformations. For example, unusually hard, infusible coatings can be produced on thin sheets of metal by exposing films of paratfin wax, and related materials, to the ionizing radiation of low voltage electrons (of the order of 50,000 volts). The procedure is described in my co-pending application S.N. 531,292 filed August 29, 1955. If materials of this nature are exposed to the action of high energy electrons, the supporting metal is quickly heated to an extent distilling oif the paraffin wax and preventing the formation of a useful co'ating. Accordingly, it is an object of the present invention to provide an electron accelerating system for exposing organic or thermally sensitive materials such as parafiin wax to the ionizing radiation of low energy electrons in a commercially feasible way.

The invention provides process and apparatus for irradiating materials with low energy electrons which is essentially characterized by generation of a beam of electrons in one evacuated enclosure and passage of the pressure). Substantial economies in the cost and operation of the system result, and a feasible system for applying a beam of electrons generated at relatively low voltage, i.e. less than about 500 kilovolts, is provided. With the differential vacuum system, no window of the conventional type is necessary.

The material to be irradiated at low voltage is handled in the form of a thin layer such as a sheet, a stream or a film. Continuous operation is provided by feeding the material, supported as necessary on a flexible sheet, wire, or grid, via vacuum sealed inlet and outlet gates into irradiating contact with the electron beam. The surface of the material to be irradiated is exposed uniformly and instantaneously to radiation by provision of means for rapidly scanning it with the electron beam.

The particulars of the invention may be illustrated by reference to the accompanying drawing which shows a highly simplified diagram in partly sectionalized form.

The electron beam is generated in evacuated enclosure 11 by heating cathode filament 12 with power supplied from transformer 13. Transformer 13 is of a conventional type with primary coil 14 connected to any convenient source of power such as the usual 6 0-cycle line voltage and secondary coil 15 for stepping up the voltage to the order of 50 to kilovolts. Chamber 11 is evacuated by means of an efficient diffusion type pump to a pressure of the order of 10* mm. mercury (preferably 10- mm. mercury or lower). The limiting fact is the tendency of theelectron discharge to are or form a corona type discharge through gas ionization. The electron beam may be focussed in the conventional manner by means of electrostatic plates, electrode rings, or by the designand geometry of the chamber. Thus, a series of insulated plates (not shown) surrounding the desired electron path at varying voltage levels may be provided as in conventional Van de Graaff type accelerators. The plates function to accelerate the electrons as well as focus the beam and restrict its cross-sectional area to the design dimension. Alternatively, the coils of a transformer, as is also well-known, can be arranged in conjunction with accelerating electrode rings to accomplish the same purpose.

High vacuum chamber 11 is connected with a second evacuated enclosure 17 by means of an elongated tube 16. Chamber 17 is evacuated by means of suitable high speed mechanical vacuum pump, and hence is at a lower vacuurn level (higher pressure) than chamber 11. The pressure in chamber 17 is not critical from the standpoint of radiation efficiency but is determined by economic considerations, e.g. balancing the savings gained by operating at higher pressure in chamber 17 against the design and operating difiiculties in maintaining a greater pressure ditferential between chambers or in modifying the vacuum required in chamber 11 for efficient electron generation. Elongated tube 16, which is shown diagrammatically and out of scale for clarity, is advantageously of a cross-sectional area which approximates, i.e. does not substantially exceed, the cross-sectional area of the electron beam directed through it. For example, with an electron beam of l to 2 centimeters diameter, the tube 16 may be of the dimensional order of 6 feet in length by 1 inch in diameter. Although the tube 16 functions to pass and direct the electron beam into radiation chamber 17, its restricted cross-section in relationship to its length resists the back passage of gas molecules from chamber 17 to chamber 11, since this can occur only by the slow process of diifusion, to an extent permitting operation at ditferent pressure conditions in the two chambers. Thus, a vacuum of about 10* or 10 mm. of mercury can be used in chamber 17.

As illustrated, chamber 17 comprises a box-like upper portion 18, of rectangular section, which is open on its Patented June 20, 1961* bottom side. The vertical walls of member 18 extend into channels 19 and 20 formed by the walls 19a, 19b, 20a and 20b and of U-shaped member 21. The channels are open to the atmosphere and contain a high density, low vapor pressure liquid such as mercury, woods metal or the like to a level sufficient to maintain a seal when vacuum is produce inside chamber 17. Now, a supply of melted wax, or other material to be irradiated, can be maintained on the surface of the liquid metal in one of the outer channels, e.g. channel 19. A sheet of metal 26, e.g. a light steel plate of about inch thick, is passed into chamber 17 by drawing over a series of rollers 27 through the supply of melted wax 28, through liquid seal 24, through the evacuated channel of chamber 17 under the outlet of tube 16 and thence out the other side via liquid seal 25. In this manner, sheet metal direct from a rolling mill can be economically coated with Wax, mixtures of wax, fatty acids such as stearic acid, asphalt, and other polymeric or resinous surface coatings and the coatings can be continuously and rapidly irradiated to increase hardness and adhesiveness. The entire operation can be conducted at any desired rate on a production line basis.

The irradiation advantageously is conducted by rapidly scanning the material passing through chamber 17 under tube 16 with the electron beam. Conventional scanning means such as electro-magnetic coils 30 may be provided to cause a rapid oscillation of the electron beam across the surface of the material undergoing irradiation. The advantage of the oscillating beam is in effecting almost instantaneous radiation of the material. In irradiating wax, for example, the more rapidly the irradiation is effected, the better is the result in terms of hardness and homogeneity of surface as well as in efficient operation of the irradiating system. If wax, or other organic materials, are exposed more than briefly to the action of the irradiating electron beam, incipient decomposition begins. The resulting gas formation, even though slight, may cause pin holes in the surface coating, and the gas generated destroys the efliciency of the evacuation, and hence may seriously interfere with the operation. The scanning rate advantageously is correlated with the speed at which the sheet or length of material to be irradiated is fed to chamber 17 so as to provide complete and uniform exposure of the surface. Although the means illustrated are limited to irradiation of a single surface, it is obvious that the system can be compounded to provide one or more additional sources of irradiating electrons to treat additional surfaces simultaneously.

In an operating example, a beam of electrons is generated at filament 12 by application of a potential of 50 kilovolts from transformer 13. A vacuum, mm. of mercury, is maintained by means of a diffusion pump. The beam of electrons is focused to a pencil-like beam, 2 cm. in diameter, and is passed through elongated tube 16 which is 6 feet by 1 inch in dimensions. The beam is caused to scan the surface of a 6 foot Wide sheet of metal which is coated with a film of paraflin wax of about 1 mil thickness. The scanning rate is 120 cycles per second. The metal is charged to chamber 17 by drawing across the series of rollers 27 through a supply of paraflin wax containing 1% stearic acid dissolved therein, which is maintained as a pool over mercury in channel 19. The more general physical and mechanical considerations relating to irradiation of surface coatings of this type, together with more detailed examples of suitable materials, are described in my above co-pending application.

The electron beam may be continuous but advantageously is of the pulsing type since alternating current is more readily available at lower cost and since the system is automatically self-rectifying. In generating the beam, more than one filament may be employed in known manner in order to speed up scanning of the surface. The design of the constricted tubular passageway connecting the generating vacuum chamber with the irradiating vacuum chamber can be varied considerably according to the size and shape of the electron beam and the design geometry of the scanning path to provide uniform treatment of the material to be irradiated. The pressure differential that can be maintained without disturbing the electron generating operation is increased by lengthening the tube or by reducing its cross-sectional area, but on the other hand the requirements for accurate focusing of the beam are increased.

Insulators 29 may be provided as indicated so as to permit maintaining a high potential on the metal enclosure or bell-dome of chamber 11 which has the advantage of permitting a higher operating pressure in chamber 11 without formation of a corona discharge. The insulators may be installed at any convenient location below the generator enclosure or dome and the remaining structure then operates at ground potential. Also, in practice, the entire accelerating apparatus may be enclosed in a metal casing or tank.

I claim:

1. Apparatus for irradiating materials with low energy electrons which comprises: a first chamber, means for maintaining in said first chamber a vacuum effective to permit the generation of a low energy electron beam therein, means positioned within said first chamber for generating a beam of low energy electrons, a second chamber, inlet and outlet passages communicating with said second chamber and adapted for continuously passing a thin layer of material to be irradiated into and out of said second chamber without loss of vacuum therein, means for rapidly scanning the surface of said material with the electron beam while said material is passing through said second chamber, means for maintaining in said second chamber a vacuum lower than the vacuum in said first chamber, and an elongated tube of restricted cross-section connecting and opening directly into said first and said second chambers to provide a passageway for the electron beam from said first chamber into said second chamber while resisting back diffusion of gases from said second to said first chamber, said elongated tube being the efifective means of providing the vacuum differential between such chambers.

2. Apparatus of claim 1 wherein said inlet and outlet passages communicating with said second chamber and adapted for continuously passing a thin layer of material to be irradiated into and out of said second chamber are vacuum sealed with a pool of high density nonvolatile inert liquid.

References Cited in the file of this patent UNITED STATES PATENTS 1,630,826 Brooks May 31, 1927 2,293,840 Lignian Aug. 25, 1942 2,504,362 Verhofi Apr. 10, 1950 2,640,948 Burrill June 2, 1953 2,729,748 Robinson Jan. 5, 1956 2,737,593 Robinson Mar. 6, 1956 2,785,313 Trump Mar. 12, 1957 2,793,282 Steigerwald May 21, 1957 2,794,847 Streuber June 4, 1957 2,887,584 Nygard May 19, 1959 FOREIGN PATENTS 161,512 Australia June 12, 1952 

